Beam Quality of Multicore Fibre Lasers

In the paper the beam quality of the phase-locked multicore fibre lasers was investigated. The beam quality factor (BQF) of the coherently combined beam of the multicore fibre lasers should be determinate as the laser optical output power in a central peak far-field bucket divided by the total optical output power radiating from the effective near-field. Classical $M^2$ factor is not proper for evaluating the beam quality of phase-locked multicore fibre lasers because it degrades with the increasing number of cores. The beam quality factor of the manufactured multicore fibres equals: 7-core hexagonal structure fibre (BQF = 0.71, V = 2.4), 5-core (BQF = 0.70, V = 2.4), 30-core circular structure of core optical fibre laser possesses the lowest beam quality factor (BQF = 0.48, V = 2.4). However, the angular divergence of the central peak is reduced in proportion to the number of cores generating mutually coherent radiation. Numerical simulation shows that standard deviation of the phase error below 15° has an inconsiderable impact on the laser beam quality. The luminescence spectra of the fabricated constructions: 5-core, 7-core and 30-core double clad multicore optical fibres doped with $Nd^{3+}$ were measured.